1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography.
2. Description of the Related Art
Roughly, image formation using electrophotography is performed through the following process: First, a photosensitive member as an image bearing member is charged by an electrostatic charger, and an invisible electrostatic latent image is formed on the surface of the charged photosensitive member by being irradiated with light by an exposure device, whereafter a toner image is generated by visualizing the invisible electrostatic latent image using colored toner particles as a developer. The so-called developing process for generating the toner image is realized by moving and placing the charged toner particles by electrostatic forces. Then, the toner image formed on the surface of the photosensitive member is transferred onto a print sheet by electrostatic forces directly or via a transfer member and is finally fixed on the print sheet by a fixing device.
In an apparatus configured to form an image by electrostatically attaching toner onto a photosensitive member, a change in the amount of charged toner (hereinafter referred to as “the toner charge amount”) directly leads to changes in color hue and density. For example, the toner charge amount changes with time according to an amount of printing of characters and images, a toner replenishment rate, an environment, and so forth, and hence even in a case where the same image is continuously printed, color hue and density can differ between a first copy and a final one. To cope with this problem, it is important to accurately grasp a change in the toner charge amount, i.e. charge-development characteristics.
To improve stability of image quality (i.e. the quality of printing on print sheets or the like), there has been proposed a technique in which a predetermined gradation patch is formed before or after image formation or during image formation and a deviation of a formed gradation patch from a proper one to be formed is corrected. For example, after completion of warm-up of an image forming apparatus, a predetermined image pattern is formed on an image bearing member, and the density of the image pattern is detected. Then, the configuration of a circuit, such as a gamma correction circuit, for changing image forming conditions is changed to improve the stability of image quality (see e.g. Japanese Patent Laid-Open Publication No. H04-343573).
However, the conventional technique for improving the stability of image quality suffers from various problems. The problems will be described with reference to FIGS. 18A to 18D. FIGS. 18A to 18D are diagrams schematically showing charge-development characteristics of an image forming apparatus that forms images using electrophotography.
FIG. 18A schematically shows the relationship between time elapsed after the start of the image forming apparatus and the toner charge amount. When the image forming apparatus is started, a developing device starts operation (rotation), and the toner charge amount rises toward a saturated charge amount. Depending on timing in which charge characteristics (toner charge amount) are acquired during the rise in the toner charge amount, a difference (deviation) can occur between the toner charge amount at the time of acquisition of the charge characteristics and a toner charge amount at the time of actual printing. The difference (deviation) seriously influences image quality.
More specifically, as schematically illustrated in FIG. 18B, when a toner charge amount set for actual printing is high, the amount of toner particles attached onto an electrostatic latent image formed based on the obtained charge characteristics is reduced, which makes output image density (print density) low. On the other hand, when the actual toner charge amount is low, the amount of toner particles attached onto an electrostatic latent image formed based on the obtained charge characteristics is increased, which makes the output image density high. Note that a vertical axis in FIG. 18B represents the surface potential of a photosensitive member, and “Vl” represents a light potential (potential in an exposed area), “Vcont” a developing contrast potential, “Vdev” a developing bias potential, “Vback” a fog removal potential, and “Vd” a dark potential.
As shown in FIG. 18C, when image forming conditions are set in a state where a difference (deviation) in toner charge amount is not corrected, control deviating from optimal gradation characteristics is performed, so that a density change from a target density is increased, which causes serious degradation of control stability. As a consequence, the difference between the target density and the output image density is increased with an increase in the number of print sheets as shown in FIG. 18D, which makes color very unstable.